This research proposal involves the elucidation of the mechanism of oncogenesis involving murine leukemia viruses (MLVs). It focuses on the transformation of T lymphocytes by lymphomagenic mink cell focus-forming (MCF) MLVs and is aimed at answering the question of how a particular transforming replication competent retrovirus is associated with a specific cell-type neoplasia. It has been shown that retroviral enhancer elements play an important role in the type of neoplasia associated with a virus and that certain enhancers have a greater activity in T cells than in other cell types. One of the specific aims of this grant is to characterize the enhancers of MLVs with different oncogenic properties to identify those sequences which are responsible for the two major activities of enhancers, i.e., transcriptional activation and tissue specificity. Both recombinant enhancers and deletion mutants will be constructed and tested in various cell types for these enhancer studies. The T cell-specific enhancer will be exploited in identifying the subclass of thymic cells which is the target for transformation by the MCF MLVs. Thymus cells will be fractionated into different subsets and the expression of a fluorescent marker in these cells will be assayed by using a fluorescence-activated cell sorter. The study of trans-acting cellular factors which could interact with sequences in the long terminal repeats of these viruses is aimed at understanding the mechanism by which these sequences are activated. The study of these factors should contribute to the understanding of how other retroviruses can efficiently replicate in and affect other T cell types. These retroviruses include HTLVIII, the AIDS-associated virus. These experiments will involve the analysis of protein-DNA complexes. Studies of MLV proviruses in thymic lymphomas are aimed at understanding how they can activate adjacent cellular genes.